Photovoltaic (PV) cells are made of many materials. Among common PV cells are those having active photon-absorber layers of polycrystalline Silicon, Single-crystal silicon, or amorphous silicon; photovoltaic cells may be made from other semiconductor materials such as Germanium, Gallium Arsenide, Gallium Phosphide, Indium Gallium Phosphide, Cadmium Telluride, Copper-Indium Gallium diSelenide (CIGS), Copper Oxide, Zinc Oxide, Zn3P2, and Indium Gallium Nitride.
It is well known that PV cells having absorber layers of single-crystal or large-crystal-grain semiconductors are more efficient at converting energy from incident photons into electrical energy than those of polycrystalline, microcrystalline, nanocrystalline, or amorphous materials due to less material defects. Here “single crystal” means the entire semiconductor material has the same crystallographic orientation without any grain boundaries. “Large-crystal-grain” means that the grain size is comparable or larger than the carrier diffusion length such that electron-hole recombination at grain boundaries is negligible. For example, single-crystal silicon cells are typically as much as twice as efficient as polycrystalline or amorphous silicon cells. However, single crystal materials are significantly more expensive than their polycrystalline, microcrystalline, nanocrystalline, or amorphous counterparts, which is a limiting factor for their applications.
Most single-crystal silicon PV cells available today are made from silicon wafers produced by growing large, single-crystal, boules using the Czochralski process, and sawing slices, or wafers, from the boule. The wafers are then polished and the photovoltaic device formed on and in the wafer. This process is considerably more expensive than forming polycrystalline, microcrystalline, or amorphous thin-film layers on a substrate such as glass. Single crystal wafers of other semiconductor materials are even more expensive than single crystal Si wafers. For example, GaAs wafers are several times more expensive than Si wafers with the same area. For some semiconductor materials such as CIGS, single crystal wafers are unavailable. Furthermore, since sawn wafers have a minimum practical thickness and material is lost from sawing, over 100 times more semiconductor material is typically required to create PV cells of a particular surface area from Czochralski wafers than from thin-films. In addition, the sawing and polishing process is expensive, and wafers tend to be stiff and brittle. Although new technologies such as direct wafer casting from molten Si are being developed to reduce material wastes associated with sawing, the wafers produced are multicrystalline instead of single-crystal. Moreover, the direct wafer casting technique is still unlikely to catch up with the low cost of thin-film layers since the wafers are still about 100 times thicker than thin-films while the material growth temperature is nearly 1000° C. higher.
Layers of polycrystalline, amorphous, and microcrystalline silicon and other semiconductor materials may be deposited on a substrate by various versions of chemical vapor deposition (CVD) and physical vapor deposition (PVD), including plasma-enhanced CVD (PECVD), low pressure CVD (LPCVD), atmosphere pressure CVD (APCVD), ultra-high vacuum CVD (UHV-CVD), thermal evaporation, electron-beam evaporation, sputtering, and laser ablation. Techniques for depositing polycrystalline silicon are well known in the integrated circuit art. Polycrystalline silicon can be deposited on a large variety of substrates, ranging from metal foils to some kinds of glasses, and may also be deposited over conductive metal films previously deposited on some glasses and similar substrates.
Amorphous, nanocrystalline or microcrystalline silicon layers may be deposited on substrates having softening temperatures of less than 600 C; laser recrystallization has been performed on such layers to produce polycrystalline silicon layers at relatively low-temperature on polyester substrates.
As previously mentioned, PV cells fabricated from single-crystal or large-grained silicon wafers are twice as efficient yet considerably more expensive than those made with polycrystalline or amorphous silicon thin films. Further, cells fabricated from Czochralski wafers are fragile, and are usually packaged in inflexible, heavy, and bulky panels for use in fixed solar panel arrays.
Single-crystal thin-films are an alternative approach to high efficiency PV cells. Conventionally, single-crystal thin films have been grown on single crystal substrates or template layers by epitaxy. However, inexpensive, non-single-crystal substrates such as glass, plastics or metal make it impossible to form single-crystal thin films by conventional epitaxial growth due to lack of single crystal substrate or template. For example, glass and plastics are amorphous, while metals are polycrystalline. Thin films deposited on these substrates are also amorphous, microcrystalline or polycrystalline since there is nothing to guide the crystallographic orientation of thin film growth. An approach to form single-crystal thin-films on non-single-crystal substrates or templates is wafer bonding using a “Smart Cut” process, a technique involving surface oxidation, hydrogen implantation into the single crystal Si wafer, bonding to the desired substrate at high temperatures, and annealing to separate a thin layer of Si from the original single crystal Si wafer. This process is very expensive and cannot be scaled to large area due to the limitation of Si wafer size (currently 12 inch in diameter). In fact, the resulting product such as silicon-on-insulator (SOI) is about 10 times more expensive than single-crystal Si wafers themselves. As a result, they are currently only used for high-end electronics and optoelectronics devices. Clearly, this approach is not applicable to high volume solar cell applications.